1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and a process cartridge for electrophotography in a copier, electrostatic printing, a printer, a facsimile, electrostatic recording, and the like.
2. Description of the Related Art
Various methods are conventionally known to form electrophotographic images. The surface of a latent electrostatic image bearing member (hereinafter, may be referred to as a “photoconductor,” an “electrophotographic photoconductor,” or an “image bearing member”) is usually charged and exposed to form a latent electrostatic image thereon. Subsequently, the latent electrostatic image is developed with a toner to form a visible image on the latent electrostatic image bearing member. This visible image is transferred onto a recording medium directly or through an intermediate transfer member, and the transferred image is fixed by application of heat and/or pressure. Thus, the image is formed on the recording medium, and a record is obtained. After the visible image is transferred, residual toner on the latent electrostatic image bearing member is removed by a known method using a blade, a brush, a roller, or the like.
In general, there are two types of full-color image forming apparatuses utilizing this electrophotography. One is called a single (or single drum) image forming apparatus. This image forming apparatus is equipped with one latent electrostatic image bearing member and four developing units for four colors—cyan, magenta, yellow, and black. Visible images of the four colors are formed on the latent electrostatic image bearing member or a recording medium. Moreover, it is possible to use the same charging unit, exposing unit, transfer unit, and cleaning unit, which are disposed around the latent electrostatic image bearing member, for each image formation. Thus, the single image forming apparatus can be designed smaller at lower costs than a tandem image forming apparatus described later.
The other apparatus is called a tandem (or tandem drum) image forming apparatus. This image forming apparatus is equipped with a plurality of latent electrostatic image bearing members (refer to Japanese Patent Application Laid-Open (JP-A) No. 05-341617). A charging unit, a developing unit, a transferring unit, and a cleaning unit are generally disposed for one latent electrostatic image bearing member to form one image forming element as a whole. The image forming apparatus is equipped with the plurality of image forming elements (generally four). One image forming element forms a visible image of one color, and visible images are sequentially transferred onto a recording medium to form a full-color image. Thus, it is possible to form an image at high speed since a visible image of each color can be formed in parallel processing. More specifically, the tandem image forming apparatus takes time four times shorter than the single image forming apparatus takes to form an image so as to print four times faster than the single image forming apparatus. Moreover, each unit (e.g., a latent electrostatic image bearing member) of the image forming element can be substantially more durable. It is because each latent electrostatic image bearing member in the tandem image forming apparatus performs a sequence of charging, exposing, developing, and transferring steps only once to form one full-color image, whereas a latent electrostatic image bearing member in the single image forming apparatus performs the sequence four times.
However, the size and costs of the tandem image forming apparatus are disadvantageously increased since the plurality of image forming elements are disposed therein.
To overcome the problems, the latent electrostatic image bearing member and each unit disposed therearound are made smaller to decrease the size of one image forming element. As a result, not only the size of the image forming apparatus, but also the material costs are reduced so that the entire costs of the image forming apparatus are lowered to some extent. However, as the image forming apparatus is made more compact and smaller, it is necessary to enhance the performance and greatly increase the stability of each unit in the image forming element.
In addition, energy-saving and high-speed image forming apparatuses such as printers, copiers, and facsimiles have been recently demanded in the market. To achieve these performances, it is important to improve thermal efficiency of a fixing unit in the image forming apparatus.
An unfixed toner image is commonly formed on a recording medium (e.g., a recording sheet, printing paper, photographic paper, or electrostatic recording paper) in an image forming apparatus by image forming process (e.g., electrophotographic recording, electrostatic recording, or magnetic recording) through indirect transfer or direct transfer. A contact heating fixing unit (e.g., a heat roller fixing unit, a film heating fixing unit, or an electromagnetic induction heating fixing unit) is widely employed to fix this unfixed toner image.
The heat roller fixing unit basically has a heat source such as a halogen lamp inside and a pair of rollers. One of the rollers is a fixing roller adjusted to be at a predetermined temperature. The other roller is a pressure roller pressured to contact the fixing roller. A recording medium is inserted into a contact portion (i.e., a nip) of the pair of rollers and transported. An unfixed toner image is fused and fixed by heat and pressure from the fixing roller and the pressure roller.
A film heating fixing unit has been proposed in Japanese Patent Application Laid-Open (JP-A) Nos. 63-313182 and 01-263679, for example. This film heating fixing unit contacts a recording medium to a heat body, which is fixed and supported by a support member, through a thin, heat-resistant fixing film. The fixing film is slit and moved along the heat body so that the heat body heats a recording medium through the fixing film.
For example, a ceramic heater is used for the heat body. This ceramic heater has a resistive layer on a ceramic substrate made of alumina, aluminum nitride, or the like. Alumina and aluminum nitride have properties such as thermal resistance, insulating properties, and good thermal conductivity. A thin fixing film with low heat capacity can be used in this fixing unit to enhance heat transfer efficiency, to shorten time for warming-up, and to enable quick-start and energy-saving, compared with the heat roller fixing unit.
A technique has been proposed for the electromagnetic induction heating fixing unit as an example (refer to JP-A 08-22206). In this technique, an alternating current magnetic field causes an eddy current in a magnetic metal material to generate Joule heat, and a heat body containing a metal material is heated by electromagnetic induction.
A film having an elastic rubber layer on its surface is disposed between the heat body and a recording medium in this electromagnetic induction heating fixing unit to cover a visible image adequately so that the image is uniformly heated and fused. When the elastic rubber layer is made of silicone rubber or the like, thermal responsiveness degrades due to its low thermal conductivity. As a result, a difference between temperatures of inner and outer surfaces of the film becomes very large. Herein, the inner surface is heated by the heat body, and the outer surface contacts the toner. When a large amount of toner is adhered, the surface temperature of a belt decreases rapidly, and the toner cannot be fixed sufficiently. This may cause cold offset.
Moreover, releasability (hereinafter, may be referred to as “anti-offset property”) of toner from the heat member is demanded during the fixing step. The anti-offset property can be improved by the presence of a releasing agent on the surface of the toner. However, when a toner (except a predetermined toner) is recycled, the amount of releasing agent is decreased on the surface of the toner. In addition, when particles designed to have a core-shell structure with two or more layers are localized in the surface of the toner, the minimum fixing temperature is increased, and low-temperature fixing property, in other words, energy-saving fixing property cannot be sufficient. Furthermore, when the toner needs to be fixed at lower temperature in a low-temperature fixing system, the toner cannot be fixed well because of fine inorganic particles localized in the surface of the toner. Therefore, a wide range of the fixing temperature could not have been obtained.
In line with the development of electrophotography, a toner needs to have excellent low-temperature fixing property, anti-offset property, and storage stability (anti-blocking property). Accordingly, various types of toner have been reported, and examples of the toner include: a toner which contains a linear polyester resin with defined physical properties such as molecular weight (refer to JP-A 2004-245854); a toner containing a non-linear cross-linked polyester resin using rosins as an acidic component of the polyester (refer to JP-A 04-70765); a toner having fixing property improved by using a rosin modified by maleic acid; and a toner using a resin made by blending a low molecular weight resin and a high molecular weight resin (refer to JP-A 02-82267).
However, as recent machines become faster and energy-saving, a conventional toner binder resin is insufficient to meet the demand in the market. More specifically, it is very difficult to maintain sufficient fixing property when fixing time is shortened during the fixing step and a heating temperature of a fixing machine is lowered. Particularly, a glass transition temperature is inevitably decreased when a low-molecular weight resin is employed. Consequently, the toner is aggregated when stored.
Moreover, when a strong stress is applied during printing, image degradation is significant under high-speed repetitive printing because of insufficient toner durability and filming due to insufficient dispersion of an internal additive.
Furthermore, when the low molecular weight resin and high molecular weight resin are blended, the pulverizability is disadvantageously inferior in a resin production process due to presence of the high molecular weight component and in a pulverized toner production process using the binder resin.
Further, rosin monomers used in JP-A 04-70765 and 04-307557 effectively improve the low-temperature fixing property while the monomers are likely to cause odor.
Therefore, it is desirable to promptly provide an image forming apparatus, an image forming method, and a process cartridge, which employ a toner having excellent low-temperature fixing property, anti-offset property, durability, pulverizability, and storage stability and causing less odor, have stability over time, and are enabled to form a high quality image for a long period of time.